1. Field of the Invention
The present invention relates to a method of forming leads of a semiconductor device to a predetermined bent shape, and more particularly to a method of forming leads of a semiconductor device such as of the QFP (Quad Flat Package) or SOP (Small Outline Package) type.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a conventional process of forming leads of a semiconductor device. As shown in FIG. 1, semiconductor device 1 packaged on a lead frame is placed in die assembly 10 by a feeder (not shown). Then, press 11 is actuated to close die assembly 10, thereby forming separate leads 12 extending from the lead frame to a predetermined shape. Specifically, when press 11 is actuated, lead holders 9 hold lead bases, or proximal lead ends, against die 8, and then punches 7 press leads 12 downwardly against die 8. Leads 12 are sandwiched between and plastically deformed to the desired shape by punches 7 and die 8. Punches 12 and die 8 are dimensioned to press leads 12 to the final shape when punches 12 are displaced toward die 8.
Conventional die assembly 10 needs to be newly manufactured and the dimensions of the pressing components have to be measured each time semiconductor devices of different package sizes are fabricated. Die assembly 10 is expensive, e.g., it costs 2 million to 4.5 million yen, and it takes a long period of time, e.g., 2 to 4 months for the die manufacturer to supply a new die assembly to the semiconductor manufacturer. The long period of time required to manufacture die assemblies has been a bottleneck in the development of new semiconductor devices.
The recent trend of the semiconductor industry is toward the fabrication of small quantities of semiconductor devices of many different types. Since the number of semiconductor devices per lot s small, e.g., several hundred or less, according to this fabrication practice, the time required to produce one lot of semiconductor devices with one die assembly is short, resulting in frequent die assembly replacement (one replacement process takes about 20 minutes). Accordingly, the availability of the semiconductor fabrication system is lowered, as is the productivity thereof. Another problem of an increased number of die assemblies used is that a large floor space is required to store the die assemblies in the factory, and hence the efficiency of factory floor utilization becomes poor.
When leads 12 are formed by die assembly 10, solder-plated layers are scraped off leads 12 because they are held in abrasive engagement with punches 7 and die 8. The scraped solder particles are attached to leads 12, tending to develop a short circuit when semiconductor device 1 is put to use.